A hydraulic circuit relies on a pump to push oil through a control valve and into an actuator, such as a cylinder or motor. The actuator typically moves a load against gravity. At times, the force required to overcome gravity substantially reduces and, in fact, gravity may take over and tend to drive the actuator via its connection to the load. This condition is commonly referred to as an “over-center” condition of the load. During an over-center condition, the load is no longer controlled by flow of fluid from the pump. As such, the load begins to “run-away” from the pump and its motion is controlled by the acceleration of gravity.
To avoid operating under such conditions, hydraulic systems often include motion control valves, which are also known as a counterbalance valves, operating to retard motion of an actuator during an over-center condition and preserve control of the load by the flow and pressure of fluid provided by the pump. In general, a counterbalance valve is a pressure control valve.
In pressure control valves, a pressure setting of the valve is proportional to the magnitude of the force of a spring acting on the active element (typically referred to as a spool) of the valve. Thus, a typical pressure control valve may include a single spring or, for valves having higher pressure settings, a series of springs directly pushing against an active element. It can be appreciated that valves that include multiple springs will be physically larger to accommodate the springs.
The active element, or spool, is disposed to selectively control a flow path interconnecting the actuator and a tank or reservoir of the hydraulic system. In the case of a counterbalance valve, the more the load tends to “run-away” from the oil supplied by the pump, the more the spring pushes on the active element in a closing direction. In turn, the flow of oil is restricted until flow from the pump matches the motion of the load.
Thus, the counterbalance valve is able to control motion of the actuator during an “over-center” condition. However, circuits using such known counterbalance valves are inefficient to operate. For example, friction caused by the restriction of oil flow causes a temperature increase of the oil. Further, power from the prime mover driving the pump (e.g. internal combustion engine) is wasted.
Under operating conditions when the load is being controlled by pump flow (a non-over-center condition), the counterbalance valve should not restrict the flow of oil therethrough. For this reason, a typical counterbalance valve includes means for reducing the effective spring setting of the spring acting upon the active element.
For example, a separate port, which is commonly referred to as a pilot port, is connected to a pilot chamber within the valve. During operation, load-induced pressure is provided to the pilot chamber. The pilot chamber is typically opposite the load pressure port and is directly linked to the moveable spool element, which is the same spool that exhausts the fluid to tank. The hydraulic area of the pilot chamber is typically larger than that of the exhaust chamber to permit motion of the spool out of the flow path in the presence of sufficient load pressure at the pilot port during non-over-center valve operation.
In known valves, a large ratio between the pilot chamber and exhaust chamber areas is desirable because it enables motion of the spool at relatively low load pressures. The large ratio also minimizes flow restriction through the valve when motion control is not required. On the other hand, a small ratio is desirable to provide system stability during over-center operation. The small ratio provides a quick response time for the valve when the load pressure decreases in response to an over-center condition.
Counterbalance valves are typically used in circuits where the flow into the actuator is controlled by a device know as a flow control. In such circuits, the counterbalance valve controls the exhaust flow. One example of such a circuit is known as a bridge circuit. Bridge circuits are inherently stable systems because of the function of the counterbalance valve(s) they include. A conventional bridge circuit arrangement can include four flow control valves, two of which are provided to control the flow of fluid into an actuator, and the other two to control the exhaust flow of fluid out of the actuator.
In known systems, the position of the actuator and pressure of the system are monitored to determine when a load has moved into an over-center condition. As the load moves into an over-center condition, the system attempts to predict the amount of restriction that should be imposed on the system. Such predictive control takes time to complete and demands computational capacity that would otherwise have been used for other functionality of the system. Moreover, system calibration is specific to a given system and must be conducted for each type of system individually.